Running rough at idle

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ValiantOne

ValiantOne

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My 273 sb, 2 bbl stock engine just started running rough at idle, and when coasting or at partial idle. Popping sometimes under trailing throttle.

Had the engine out last winter. Replaced all gaskets (intake incl) distributor, cap, rotor, wires, plugs. Upgraded with Trailbeast's hei upgrade. All was good! Ran like a champ for a few months. Now experiencing the problem above.

I widened the plug gap a few weeks back to .045, for the hei, I put it back to .035 today. No change. I tried reversing the distributor signal leads when I installed the hei. Ran crappy one way so i installed them the other way as instructed.

Just did the plug wires this weekend. No change.

I'm getting just at 12.7v to the coil.

The left bank exhaust smells gassier than the right. Seems ignition related. Not sure what to check next?? Maybe plug wire resistance?

Any thoughts?

Thanks

CE
 
Question.. Do you have power brakes ? If so,,My ol'Ram truck started doing that,,Stopped fine but it was a leak in the booster that was leaning out a cylinder. Just a thought......
 
Darter6 said:
Question.. Do you have power brakes ? If so,,My ol'Ram truck started doing that,,Stopped fine but it was a leak in the booster that was leaning out a cylinder. Just a thought......
Click to expand...
No power brakes. Replaced all vacuum lines and caps.

Hmm, gonna go check the carb bolts. I had thought about a vacuum leak, but with one bank smelling gassy and the other normal i just assumed ignition.

Thanks
 
Turn your idle mixture screws all the way in counting the turn until they lightly stop.
Remove them completely, shoot a little carb cleaner or WD40 in the holes and blow air through them right away.
(as in don't let the liquid sit and dry up) as you need that in there when you blow air through them to carry junk out.

Put the needles back in and lightly bottom them out like when you screwed them in and counted the turns.
Back them out to the same amount of turns as they were.

Bet that does it, as you probably have something in the idle circuit making one bank run lean at idle (popping on decel and rough idle)
 
TrailBeast said:
Turn your idle mixture screws all the way in counting the turn until they lightly stop.
Remove them completely, shoot a little carb cleaner or WD40 in the holes and blow air through them right away.
(as in don't let the liquid sit and dry up) as you need that in there when you blow air through them to carry junk out.

Put the needles back in and lightly bottom them out like when you screwed them in and counted the turns.
Back them out to the same amount of turns as they were.

Bet that does it, as you probably have something in the idle circuit making one bank run lean at idle (popping on decel and rough idle)
Click to expand...

Okay, trying that next!
 
I checked the carb bolts. All good. Shot some carb cleaner around the intake surfaces, all good. Shot some around carb base, all good. Shot some at the throttle rod and got a slight rpm increase. I think I have a bit of a leak there. Wouldnt expect it to show up all at once though. Hmmm.
 
ValiantOne said:
I checked the carb bolts. All good. Shot some carb cleaner around the intake surfaces, all good. Shot some around carb base, all good. Shot some at the throttle rod and got a slight rpm increase. I think I have a bit of a leak there. Wouldnt expect it to show up all at once though. Hmmm.
Click to expand...

A little around the carb shaft is normal.
I don't think you have a vacuum leak but a plugged or dirty idle circuit, so do what I suggested above.
 
Okay, tried the idle circuit bit and there was no change. But.......

A friend who is a great mechanic and an old school mopar guy himself stopped by. He and I made some mixture and timing changes and it helped.

It isn't perfect but about 75% better. Seems like it loads up a bit and gets rough when it does, but otherwise smooths out and isn't stumbling over itself as much. I think it is a bit too rich right noe and I'm going to try dialing it back some and see how it goes.

He suggested that there may be some crap floating around in the carb too.

Maybe it was a lean condition, but i don't get how it cropped up instantly.

Oh and he was APPALLED by my GM ignition :D lol
 
ValiantOne said:
Okay, tried the idle circuit bit and there was no change. But.......

A friend who is a great mechanic and an old school mopar guy himself stopped by. He and I made some mixture and timing changes and it helped.

It isn't perfect but about 75% better. Seems like it loads up a bit and gets rough when it does, but otherwise smooths out and isn't stumbling over itself as much. I think it is a bit too rich right noe and I'm going to try dialing it back some and see how it goes.

He suggested that there may be some crap floating around in the carb too.

Maybe it was a lean condition, but i don't get how it cropped up instantly.

Oh and he was APPALLED by my GM ignition :D lol
Click to expand...

I was wondering about it being rich and loading up, and that's pretty easy to diagnose because it clears up after a few good revs and then comes back within a couple of low RPM minutes.
It's totally normal for something to get crap in it and immediately start showing a problem.
Happens all the time, because it's not like something that wears out and starts working worse and worse, but more like a golf ball getting stuck in a pipe.
Everything is fine till it gets stuck, and then suddenly there is a problem.

Oh, and your friend can be appalled while he's looking at your tail lights getting smaller while he's trying to figure out where to get a ballast resistor from. :D
 
You can try clamping the rubber fuel line with the engine idling. As the fuel level in the bowl drops, one of three things will happen; If the level was too high, it will soon start to run better.If it was too low, it will run worse,almost right away. If the level was just right, it will make no difference for about a half a minute, and then run progressively worse.
Unless there is water coming up the pipe; then the problem comes and goes when driving as the water sloshes around and finds it's way into and through the mainwells. If it happens at idle, it will stall.
 
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